1. Field of the Invention
The present invention relates to an image processing apparatus, an image processing method, a program and an image forming apparatus for performing a density adjustment of an image automatically.
2. Description of Related Art
An image processing apparatus to adjust the density of a read image and to output the adjusted image as an image for image formation is known as an image processing apparatus to be built in an image forming apparatus such as a copying machine. When a kind (such as a “photograph” or a “letter”) of an image is selected and the image is read, the conventional image processing apparatus built in an image forming apparatus performs a density adjustment of the read image in accordance with a γ curve corresponding to the selected kind of the image.
For example, the following image processing apparatus is known. That is, the image processing apparatus produces a histogram of density frequencies on a read image. Then, the image processing apparatus (1) detects a highlight part by detecting the values pertaining to a foot value, a peak value and a start point (hereinafter these values will be collectively referred to as “highlight part specification information”) of a high luminance part of the produced histogram, or (2) detects a shadow part by detecting the values pertaining to a foot value, a peak value and a start point (hereinafter these values will be collectively referred to as “shadow part specification information”) of a low luminance part of the histogram, on a basis of the produced histogram and the selected kind of the image. Then, the image processing apparatus reads a γ curve corresponding to the respective detected values, and performs a density adjustment corresponding to the read γ curve.
Moreover, as disclosed in JP-Tokukaihei-11-196275A and JP-Tokukaihei-11-196276A, a technique which produces a histogram on a read image and detects characteristic points (hereinafter referred to as “character points” suitably) of the produced histogram to judge the kind of the image on a basis of the character points is also known.
However, such conventional image processing apparatus perform the density adjustment by selecting a γ curve on a basis of the values of either the highlight part or the shadow part, both detected from the histogram of a read image.
For example, when the kind of an original is selected as a “letter”, such a conventional image processing apparatus selects a γ curve corresponding to a highlight part of a histogram to perform a density adjustment. Hereupon, when the image processing apparatus reads an original of letters having a dark background, the image processing apparatus selects a γ curve for performing a light density expression for lightening the background. Consequently, the image processing apparatus has a problem of making letters also light.
Moreover, because an image processing apparatus which automatically judges the kind of a read image performs the judgment only on a gradation at the time of judging an image, the image processing cannot always perform a proper image judgment. Consequently, the image processing apparatus has a problem of the performance of a improper density adjustment owing to an erroneous judgment.
Moreover, in the technique disclosed in JP-Tokukai-2003-51938A, a histogram distribution is obtained, and an image judgment is performed by using a function coefficient approximate to the distribution as a parameter.
However, because this method performs the judgment by focusing attention only on the distribution, the judgment is not performed correctly in case of some gradation widths.